Frequency Division Multiplexing (FDM) is a technology, widely used in communication systems, which allows for the transmission of multiple signals simultaneously over a single transmission path. Each signal travels within its own unique frequency range, or carrier, which is modulated by the data (i.e., text, voice, video, etc.) to represent the information being transmitted.
Orthogonal Frequency Division Multiplexing (OFDM) is a technique which distributes data over several carriers that are spaced apart at precise frequencies. This spacing provides the orthogonality in this technique which prevents the demodulators from seeing frequencies other than their own.
From a frequency perspective, a typical OFDM packet structure 100 is divided into subcarriers, as shown in FIG. 1. These subcarriers are further classified into pilots (Pn) or data (Xn). Pilots are subcarriers whose value is known prior to the transmission of the packet. Knowing the value of the pilot subcarriers serves as a useful reference to estimate impairments throughout a packet.
In contrast to the pilot subcarriers, the value of data subcarriers is not known prior to the transmission of the packet. The data subcarriers hold the information which is to be transmitted, and is therefore unknown prior to transmission. Each data carrier has an associated pilot, which is typically the pilot that is closest to it in frequency. FIG. 1 shows the four pilots (P0-P3) and the associated data subcarriers for a 802.11 OFDM.
From a time perspective, the OFDM packet 200 is divided into a sequence of OFDM symbols, as is shown in FIG. 2. At the start of the OFDM packet 200 is a training symbol 201, which is followed by a number of payload symbols 203(1)-203(n). All subcarriers (pilots and data) in the training symbol 201 are known prior to the transmission of the OFDM packet, wherein, only the pilots are known prior to transmission in the payload symbols 203.
In order to distribute and transmit the various OFDM symbols, OFDM systems may employ a Quadrature Amplitude Modulation (QAM) scheme. A QAM scheme conveys data by changing or modulating the amplitudes of two carrier waves, in response to a data signal. A typical OFDM transmitter and receiver pair 300 employing a QAM scheme is shown in FIG. 3. A flow of bits to be transmitted from a digital transmitter baseband 304 is split into two equal parts, therefore generating two independent signals. The two signals are converted to analog signals via digital to analog converters (DAC) 306a and 306b. The analog signals are then passed through low pass filters (LPF) 308a and 308b. The two analog signals are then encoded separately, with one analog signal being multiplied 308a by a cosine wave and the other analog signal being multiplied 308b by a sine wave. The encoded signals are then added together 311, and the combined signal is then passed through a power amplifier 312 and transmitted with the use of an antenna 313. The receiver 302 detects the transmitted signal with the use of an antenna 315 and transmits the signal to a radio frequency receiving device 314. The received signal is then separated and decoded with the multiplication of a cosine wave 316a and a sine wave 316b. The signals are then passed through a low pass filter 318a and 318b, and thereafter, the signals are digitally transformed with the use of analog to digital converters 320a and 320b. Finally, the signals are then sent to the digital receiver baseband 322.
In the radio frequency (RF) portion of the transmitter, electromagnetic waves are generated and transmitted by alternating current fed into antenna 313. This RF section is also a region that may be susceptible to gain variations. Gain variations may be caused by a number of factors, one of which may be thermal changes in the transmitting device. FIG. 4 shows a calculated transient electrical response of a self-heating device. The self-heating effect, caused by a temperature rise due to a power dissipation and a temperature dependence of device characteristics, can be regarded as a thermal-electrical feedback inside the device (f). The y-axis of the plot represents the gain change of the device and the x-axis represents normalized time.
As may be seen in FIG. 4, the device is stable, or endures minimal gain variations with respect to time, when f<1. The device becomes unstable, or endures sustainable variations with respect to time, when f≧1. The graphical data illustrates gain changes increasing linearly with time for f=1, and exponentially with time for f>1.
The effects of gain variations on a signal are also shown in FIGS. 5A and 5B. In an undistorted QAM constellation, points are usually arranged in a square grid with equal vertical and horizontal spacing and with the number of points on the grid typically being a power of 2, as may be seen in FIG. 5A. FIG. 5B displays a QAM 64 constellation, which may be used to support 48 and 54 Mbps rates in a typical 802.11 transmitter and receiver pair, with a 0.5 dB gain distortion. As is shown in FIG. 5B, the 0.5 dB gain distortion has caused the constellations of the QAM to become blurred and expanded. Distortions may include impairments such as group delay, discrete echoes (e.g., reflections, ghosts or multipath distortion), micro-reflections and amplitude tilt. Distortions such as the ones previously mentioned are capable of causing serious damage to upstream data. The table below records the signal to noise ratio (SNR) limit that is imposed on a signal due to gain variation or distortion.
PowerAmplitudeSNRVariationVariationLimit(dB)(dB)(dB)0.10.0538.70.20.1032.70.30.1529.10.40.2026.50.50.2524.50.60.3022.90.70.3521.50.80.4020.30.90.4519.21.00.5018.3
As this table shows, a 0.25 dB gain variation in amplitude and a 0.5 variation in power will impose a 24.5 dB upper bound on the SNR. A QAM 64 or a 54 Mbps system may not function properly with such bounds since this system generally needs 27-28 dB in a typical indoor environment. Also, the 802.11 standard transmitter and receiver pair mandates a minimum error vector magnitude (EVM) of 25 dBc. With gain changes of 0.25 dB or more, the transmitter will not even meet the EVM specification (assuming that the transmitter is the source of gain variation).
Prior art methods of gain compensation have been developed in an attempt to correct the distortion caused by gain variation. Many prior art methods of gain compensation make use of the training symbols in the OFDM packet to estimate the gain. An estimation may typically be made at a time 0, or once a packet is first received. Thus, many prior art systems will evaluate the pilots of the training symbol once the packet has first arrived, ‘time zero,’ and estimate a gain variation. The data subcarriers of the payload symbols are then compensated based on the initial estimation made at ‘time zero.’ An updated estimation may be made, again at time zero, once a new packet has been received.